Thankfully the old slogan Ulster Says No is long past its sell-by date but yesterday a new one emerged - the DUP Says No.

What an extraordinary relationship Theresa May appears to have with the party which is keeping her in power.

The fact that the Prime Minister was about to sign up in Brussels to a form of words on Northern Ireland which had not been agreed - had it even been discussed? - with Arlene Foster begs an obvious question. Are they on the same wavelength at all?

How come that at the 11th hour of such vital negotiations over Brexit and the border, after weeks of wheeling and dealing, the prospects of agreement fell at lunchtime on the strength of a last-minute telephone call between May and Foster?

The sight of Foster and her DUP colleagues parading down the stairs into Stormont's Great Hall to effectively veto what May was about to accept in Brussels raises the question of what trust, what empathy, really exists between the Tory hierarchy and the DUP?

On the evidence of yesterday, not a lot. And if that is the case, the DUP needs to tread warily as the history of Northern Ireland has shown how many times previous unionist faith in London governments has been misplaced.

Of course, the DUP can say it couldn't happen since no unionist party before has held such a whip hand at Westminster.

After all, the future of Theresa May is in the hands of the party's 10 MPs, so how could she possibly go against their interests.

She didn't yesterday, choosing to think again after hearing the thoughts of Arlene Foster, but now the ball is back in both their courts. The pressure on the DUP cannot be underestimated.

On the brink of passing all the major tests in stage one of its negotiations with the European Union, the UK is being held up by the apparent obstinacy of Northern Ireland MPs. As the BBC's political editor Laura Kuenssberg reported: 'Hopes dashed by the small northern Irish party.'

The stakes are raised. Northern Ireland is now centre stage in Brexit and it remains to be seen whether the British public sympathise with the DUP position or simply say to themselves: 'Surely not that place again.'

The history of relations between British governments and unionist leaders over the past 50 years does not augur well for Arlene Foster and the DUP. Time and again, unionists put their trust in Prime Ministers only to find it misplaced. When the chips were down in negotiations, the phrase 'perfidious Albion' crossed many a unionist mind.

Perfidious Albion? The term has been used over the centuries in the context of Britain's international relations and diplomacy. It implies acts of diplomatic sleight of hand, duplicity and treachery, in respect of promises made or alliances formed with other states by British governments in pursuit of self-interest

Virtually every unionist leader since the 1960s has not got his own way with governments he may have trusted in London. Conservative leaders, in particular, have a record of hanging unionists leaders out to dry.

Brian Faulkner went to London in 1972 expecting the continued support of Edward Heath, but after a nice lunch in which they discussed yachting on Strangford Lough, Heath shocked Faulkner by pulling the plug on Stormont and, as a consequence, plunged Northern Ireland into decades of direct rule.

In 1985, James Molyneaux, who regarded himself as a close confidant of Margaret Thatcher, didn't see the Anglo-Irish Agreement coming. When Thatcher came to Hillsborough to sign the agreement with the then Taoiseach, Garret FitzGerald, the unionists were left outside the gates and in the cold for years.

More lately, even David Trimble, for all his Nobel Prize-winning role in the Good Friday Agreement, was let down by Prime Minister Tony Blair. The letter Blair wrote to Trimble promising the delivery of IRA decommissioning turned out to be hardly worth the paper it was written on.

Today, the DUP wakes up to an outside world which almost certainly does not understand the nuances of Northern Ireland. Many in Great Britain may even wonder what all the fuss is about and wonder if the politicians in Ulster are protesting unduly or overplaying their hand.

We, who live here, may understand the complexities of politics on this island and see the difference between a border in the Irish Sea and one between Tyrone and Monaghan. The question, however, is how this will all play out in the corridors of power in London and Brussels.

The next few days will be a major test for the DUP leadership in terms of handling the British and international media.

Arlene Foster and Nigel Dodds are in the eye of a European storm. Can they demonstrate that they are more than a small band of parochial, peripheral 'northern Irish' politicians holding up progress on a Brexit which ironically they voted for?

Dodds looks far more comfortable in front of the cameras than Foster. Not for nothing was he awarded the title 'Negotiator of the Year' at Westminster. He appears the key figure in what is really a make or break situation for the DUP, which must be careful not to overplay its hand.

Three words, which were disputed in yesterday's British Brexit submission, are concentrating minds on all sides: 'Continued regulatory alignment'. They appear to mean that after the UK exits the European Union, Northern Ireland would continue to comply with European regulations while Britain would not.

Does this amount to a border down the Irish Sea between Northern Ireland and Great Britain? The DUP certainly believes so. As the party which represents unionist interests in the Brexit talks, the DUP needs now to provide more clarity on how an open border with the Republic can be achieved because many people in Northern Ireland and certainly in the Republic and in Europe remain sceptical.

In the propaganda battle which wages between London, Dublin and Brussels, it is time for the DUP to stand up to the mark.